if oyster harvesting was confined to colder months when Vibrio counts in water are lowest (Twedt et al., 1981).

If it is assumed that most environmental non-O1 V. cholerae strains are nonpathogenic, identification of these strains in oysters has limited public health utility. It would clearly be of value if pathogenic strains could be differentiated from those that are nonpathogenic. Basic research in this area should be encouraged.

Vibrio parahaemolyticus

The CDC reported V. parahaemolyticus as the most common cause of vibrio disease due to consumption of seafoods from 1978 to 1987, whereas NETSU reported lower incidences (Tables 3-2 and 3-3). This is a mildly halophilic vibrio commonly isolated from fish, shellfish, and other marine sources in inshore waters, which is most abundant when water temperatures exceed 15°C. It is difficult to isolate during cold winter months. The ability to cause human gastroenteritis is most highly correlated with the production of a heat-stable hemolysin (Miyamoto et al., 1969). Most strains isolated from the marine environment lack this hemolysin and are probably not pathogenic, although nonhemolytic strains have recently been associated with illness occurring along the U.S. Pacific Coast (Abbott et al., 1989; Kelly and Stroh, 1989). V. parahaemolyticus reproduces very rapidly at temperatures of 20°C and above, and has been shown to reach potentially infective levels [more than 105 colony forming units (CFU)] in shrimp and crabs held for 2-3 hours at such temperatures (Liston, 1973). However, it is heat sensitive and rapidly killed at 60°C.

Epidemiology and risk assessment

V. parahaemolyticus is a common marine isolate, with isolation reported from water, sediment, suspended particulates, plankton, fish, and shellfish (Joseph et al., 1983). However, it is likely that only a small fraction of marine isolates are potentially pathogenic. For example, in a study by Thompson and Vanderzant (1976), only 4 of 2,218 isolates from Galveston Bay were able to produce the heat-stable hemolysin generally associated with virulence.

In Japan, V. parahaemolyticus has been implicated as the etiologic agent in 24% of reported cases of food-borne disease (Miwatani and Takeda, 1976). In the United States, V. parahaemolyticus has caused several major food-borne disease outbreaks (Barker, 1974). The CDC food-borne surveillance data from 1978 to 1987 (Tables 3-2 and 3-3) reported V. parahaemolyticus as the cause of 15 outbreaks associated with 176 cases of mostly crustacean shellfish-associated illness (CDC, 1989). No other seafood-associated illnesses from V. parahaemolyticus were reported to CDC during this time; NETSU reported 52 cases in the same period (Rippey and Verber, 1988) (Tables 3-2 and 3-3).

Outbreaks of V. parahaemolyticus have often been associated with cross-contamination or time/temperature abuse of cooked seafood. Although sporadic cases associated with the consumption of raw oysters have occurred, there does not appear to be the same strong association with consumption of raw oysters as reported



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